Method and apparatus for transmitting uplink data using multiple serving cells

ABSTRACT

Disclosed are a method and an apparatus for transmitting uplink data by means of multiple serving cells. A method for a terminal for transmitting uplink data by means of multiple serving cells may comprise the steps of: the terminal receiving a first timing advance command (TAC) for a first serving cell and a second TAC for a second serving cell; and determining whether the terminal transmits uplink data by means of the second serving cell on the basis of whether the timing difference is below the threshold value, wherein the timing difference is acquired on the basis of the first TAC and the second TAC, and the first serving cell can be a cell configured so that an uplink can be always transmitted regardless of the timing difference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/778,995, filed on Sep. 21, 2015, now U.S. Pat. No. 9,769,822, whichis the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2014/002728, filed on Mar. 31, 2014, which claimsthe benefit of U.S. Provisional Application Nos. 61/808,199, filed onApr. 3, 2013 and 61/809,422, filed on Apr. 8, 2013, the contents ofwhich are all hereby incorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method and an apparatus for transmitting uplink data.

Related Art

Long-Term Evolution (LTE) based on 3rd Generation Partnership Project(3GPP) Technical Specification (TS) Release 8 is a leadingnext-generation mobile communication standard.

As set forth in 3GPP TS 36.211 V8.7.0 (May 2009) “Evolved UniversalTerrestrial Radio Access (E-UTRA); Physical Channels and Modulation(Release 8),” physical channels in LTE may be classified into downlinkchannels, such as physical downlink shared channel (PDSCH) and physicaldownlink control channel (PDCCH), and uplink channels, such as physicaluplink shared channel (PUSCH) and physical uplink control channel(PUCCH). The PUCCH is an uplink control channel used for transmittinguplink control information, such as a hybrid automatic repeat request(HARQ) acknowledgement (ACK)/non-acknowledgement (NACK) signal, achannel quality indicator (CQI) and a scheduling request (SR).

Meanwhile, an evolution of 3GPP LTE, LTE-Advanced (LTE-A), isdeveloping. 3GPP LTE-A adopts carrier aggregation. Carrier aggregationuses a plurality of component carriers. A component carrier is definedby center frequency and bandwidth. In carrier aggregation, a pluralityof component carriers corresponds to a single cell. A user equipment(UE) provided with a service using a plurality of downlink componentcarriers may be interpreted as being provided with the service from aplurality of serving cells. Uplink synchronization of a UE in the caseof using carrier aggregation may be difference uplink synchronization ofa UE in the case of not using carrier aggregation.

To reduce interference between UEs due to uplink transmissions, it isimportant for a base station to maintain uplink time alignments of theUEs. A UE may be located at a random place within a cell and an arrivaltime an uplink signal transmitted from a UE takes to reach the basestation may vary depending on the location of each UE. A UE located at acell edge has a longer arrival time than a UE located in a cell center.On the contrary, the UE located in the cell center has a shorter arrivaltime than the UE located at the cell edge.

To reduce interference between UEs, it is necessary that a base stationschedules uplink signals transmitted by UEs in a cell to be receivedwithin each time boundary. The base station needs to properly adjusttransmission timings of the respective UEs according to situations ofthe respective UEs and such adjustment is referred to as uplink timealignment. A random access process is one of processes for maintaininguplink time alignment. A UE acquires a time alignment value (alsoreferred to as a timing advance (TA) value) through the random accessprocess and maintains uplink time alignment by applying the timealignment value. As described above, when carrier aggregation isperformed in 3GPP LTE-A, a procedure that a UE performs uplink timealignment and uplink transmission may be different from that in the caseof performing no carrier aggregation.

SUMMARY OF THE INVENTION

An aspect of the present invention is to provide a method oftransmitting uplink data using a plurality of servicing cells.

Another aspect of the present invention is to provide an apparatus fortransmitting uplink data using a plurality of serving cells.

To achieve an aspect of the present invention, an uplink transmissionmethod of a user equipment (UE) using a plurality of serving cellsaccording to one embodiment of the present invention may includereceiving, by the UE, a first timing advance command (TAC) for a firstserving cell and a second TAC for a second serving cell and determiningwhether the UE transmits uplink data through the second serving cellbased on whether a timing difference is a threshold or less, in whichthe timing difference may be acquired based on the first TAC and thesecond TAC, and the first serving cell may be configured to alwaysperform uplink transmission regardless of the timing difference.

To achieve another aspect of the present invention, a UE for performinguplink transmission using a plurality of serving cells according to oneembodiment of the present invention may include a radio frequency (RF)unit configured to transmit and receive a radio signal and a processorselectively connected to the RF unit, in which the processor may beconfigured to receive a first TAC for a first serving cell and a secondTAC for a second serving cell and to determine whether the UE transmitsuplink data through the second serving cell based on whether the timingdifference is a threshold or less, the timing difference may be acquiredbased on the first TAC and the second TAC, and the first serving cellmay be configured to always perform uplink transmission regardless ofthe timing difference.

A user equipment (UE) determines different uplink data transmissionmethods depending on a timing advance for each serving cell inperforming uplink transmission using a plurality of serving cells,thereby enhancing uplink data transmission efficiency of the UE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a structure of a radio frame in a Long Term Evolution(LTE) system.

FIG. 2 illustrates an example of a resource grid for a downlink slot.

FIG. 3 illustrates a structure of a downlink subframe.

FIG. 4 illustrates a structure of an uplink subframe in 3rd GenerationPartnership Project (3GPP) LTE.

FIG. 5 is a schematic view illustrating multiple carriers in anLTE-Advanced (LTE-A) system.

FIG. 6 is a flowchart illustrating a random access process in 3GPP LTE.

FIG. 7 illustrates an example of a random access response.

FIG. 8 is a schematic view illustrating differences in propagationproperties between a plurality of cells.

FIG. 9 is a schematic view illustrating uplink transmission of a userequipment (UE) based on a plurality of timing advance (TA) values.

FIG. 10 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 11 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention

FIG. 12 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 13 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 14 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention

FIG. 15 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 16 is a block diagram illustrating a wireless communication systemaccording to an embodiment of the present specification.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

A wireless device may be fixed or mobile, and may be referred to asanother terminology, such as a user equipment (UE), a mobile station(MS), a mobile terminal (MT), a user terminal (UT), a subscriber station(SS), a personal digital assistant (PDA), a wireless modem, a handhelddevice, a terminal, a wireless terminal, etc. Also, the wireless devicemay be a device that supports data communication only such as amachine-type communication device.

A base station (BS) is generally a fixed station that communicates withthe wireless device and may be referred to as another terminology, suchas an evolved Node-B (eNB), a base transceiver system (BTS), an accesspoint, etc.

FIG. 1 shows the structure of a radio frame in 3GPP LTE.

It may be referred to Paragraph 5 of “Technical Specification GroupRadio Access Network; Evolved Universal Terrestrial Radio Access(E-UTRA); Physical channels and modulation (Release 8)” to 3GPP (3rdgeneration partnership project) TS 36.211 V8.2.0 (2008 March).

Referring to FIG. 1, the radio frame includes 10 subframes 120, and onesubframe includes two slots 140. The radio frame may be indexed based onslot 140, that is, from slot #0 to #19 or may be indexed based onsubframe 120, that is, from subframe #0 to subframe #9. For example,subframe #0 may include slot #0 and slot #1.

A time taken for transmitting one subframe 120 is called a transmissiontime interval (TTI). The TTI may be a scheduling basis for a datatransmission. For example, a radio frame may have a length of 10 ms, asubframe may have a length of 1 ms, and a slot may have a length of 0.5ms.

One slot 140 includes a plurality of orthogonal frequency divisionmultiplexing (OFDM) symbols in a time domain and a plurality ofsubcarriers in a frequency domain. In LTE, a BS uses OFDMA as an accessmethod in downlink channel. The OFDM symbols are used to express asymbol period, and may be called by other names depending on amultiple-access scheme. For example, in an uplink channel in which awireless device transmits data to a BS, a single carrier-frequencydivision multiple access (SC-FDMA) may be used. The symbol section inwhich data is transmitted through uplink channel may be referred to as aSC-FDMA symbol.

The structure of radio frame 100 introduced in FIG. 1 is an embodimentfor the frame structure. Accordingly, new radio frame format may bedefined by changing the number of subframes 120, the number of slots 140included in the subframe 120, or the number of OFDM symbols included inthe slot 140.

In the radio frame structure, the number of symbols included in a slotmay be changed depending on which cyclic prefix (CP) is used. Forexample, when the radio frame uses a normal CP, one slot may includeseven OFDM symbols. When the radio frame uses an extended CP, one slotmay include six OFDM symbols.

The wireless communication system may be divided into a frequencydivision duplex (FDD) scheme and a time division duplex (TDD) scheme.According to the FDD scheme, an uplink transmission and a downlinktransmission may be performed based on different frequency bands.According to the TDD scheme, an uplink transmission and a downlinktransmission may be performed based on the same frequency band by usingtime division scheme. A channel response of the TDD scheme issubstantially reciprocal since it uses the same frequency band. That is,in TDD scheme, a downlink channel response and an uplink channelresponse are almost the same in a given frequency band. Thus, theTDD-based wireless communication system may obtain the channel stateinformation from the channel state information of uplink channel. In theTDD scheme, the entire frequency band is time-divided for uplink anddownlink transmissions, so a downlink transmission by the BS and anuplink transmission by the wireless device cannot be simultaneouslyperformed.

FIG. 2 shows an example of a resource grid of a downlink slot.

A downlink slot includes a plurality of OFDM symbols in the time domainand NRB number of resource blocks in the frequency domain. The NRBnumber of resource blocks included in the downlink slot may bedetermined depending upon a downlink transmission bandwidth which isconfigured in a cell. For example, in an LTE system, NRB may have anyone value of 60 to 110 depending upon the transmission bandwidth whichis used. One resource block 200 includes a plurality of subcarriers inthe frequency domain. An uplink slot may have the same structure as thatof the downlink slot.

Each element on the resource grid is called a resource element 220. Theresource elements 220 on the resource grid can be discriminated by apair of indexes (k,l) in the slot. Here, k (k=0, . . . , NRB×12−l) is asubcarrier index in the frequency domain, and l (l=0, . . . , 6) is anOFDM symbol index in the time domain.

Herein, one resource block 200 may include 7×12 resource elements madeup of seven OFDM symbols in the time domain and twelve subcarriers inthe frequency domain. Such a size is just an example, and the number ofOFDM symbols and subcarriers constituting one resource block 200 may bechanged. The resource block pair indicates a resource basis thatincludes two resource blocks.

As described above, the number of OFDM symbols in one slot may havedifferent values depending on the CP. Also, the number of resourceblocks included in one slot may be changed depending on the size ofoverall frequency bandwidth.

FIG. 3 shows the structure of a downlink subframe.

A downlink subframe 300 may be distinguished into two slots 310 and 320base on the time domain. Each of the slots 310 and 320 includes sevenOFDM symbols in the normal CP. A resource region that corresponds tofirst three OFDM symbols (maximum four OFDM symbols with respect to a1.4 MHz bandwidth) of a first slot 310 in the subframe 300 may be usedas a control region 350 to which control channels are allocated. Theother remaining OFDM symbols may be used as a data region 360 to which atraffic channel such as a physical downlink shared channel (PDSCH) isallocated.

The PDCCH may be a control channel that transmits, for example, atransmission format and a resource allocation of a downlink sharedchannel (DL-SCH), resource allocation information of an uplink sharedchannel (UL-SCH), paging information on a PCH, system information on aDL-SCH, a resource allocation of an higher layer control message such asa random access response transmitted via a PDSCH, a set of transmissionpower control commands with respect to individual wireless devices in acertain UE group, an activation of a voice over internet protocol(VoIP), and the like. A plurality of bases that transmits the PDCCH datamay be defined in the control region 350. A wireless device may obtaincontrol data by monitoring the plurality of bases that transmits thePDCCH data. For example, the PDCCH data may be transmitted to a wirelessdevice based on one or an aggregation of a plurality of consecutivecontrol channel elements (CCE). The CCE may be a basis of transmittingthe PDCCH data. The CCE may include a plurality of resource elementgroups. The resource element group is a resource basis that includesfour usable resource elements.

The BS determines a PDCCH format according to a DCI to be transmitted tothe wireless device, and attaches a cyclic redundancy check (CRC) to theDCI. A unique radio network temporary identifier (RNTI) is masked on theCRC according to the owner or the purpose of the PDCCH. In case of aPDCCH for a particular wireless device, a unique identifier, e.g., acell-RNTI (C-RNTI), of the wireless device, may be masked on the CRC.Or, in case of a PDCCH for a paging message, a paging indicationidentifier, e.g., a paging-RNTI (P-RNTI), may be masked on the CRC. Incase of a PDCCH for a system information block (SIB), a systeminformation identifier, e.g., a system information-RNTI (SI-RNTI), maybe masked on the CRC. In order to indicate a random access response,i.e., a response to a transmission of a random access preamble of thewireless device, a random access-RNTI (RA-RNTI) may be masked on theCRC.

FIG. 4 is a view illustrating the structure of an uplink subframe in3GPP LTE.

The uplink subframe may be divided into a control region allocated to aphysical uplink control channel (PUCCH) for delivering uplink controlinformation and a data region allocated to a physical uplink sharedchannel (PUSCH) for delivering user data. PUCCH resources for allocationmay be located at the edge of bandwidth of a component carrier (CC).

The PUCCH may be allocated based on a RB pair in the subframe. RBscorresponding to the RB pair may be allocated to different subcarriersin a first and a second slots respectively. m is a position indexindicating the position of a logical frequency domain of the RB pairwhich is allocated to the PUCCH in the subframe. RBs having the samevalue of m are allocated to different subcarriers of the first andsecond slots.

According to 3GPP TS 36.211 V8.7.0, the PUCCH may have various formats.It is possible to use Different PUCCH formats with different bit numbersin the subframe according to a modulation scheme for use in the PUCCHformat.

Table 2 shows an example of bit numbers per subframe and the modulationscheme according to the PUCCH format.

TABLE 2 PUCCH format Modulation scheme bit number per subframe 1 N/A N/A1a BPSK 1 1b QPSK 2 2 QPSK 20 2a QPSK + BPSK 21 2b QPSK + BPSK 22 3 QPSK48

PUCCH format 1 for scheduling request (SR) transmission, PUCCH format1a/1b for transmitting an ACK/NACK signal for HARQ, PUCCH format 2 forCQI transmission, and PUCCH format 2a/2b for simultaneous transmissionof the CQI and the ACK/NACK signals are used. When only the ACK/NACKsignal is transmitted in the subframe, PUCCH format 1a/1b is used, andwhen only the SR is transmitted, PUCCH format 1 is used. When the SR andthe ACK/NACK signal are transmitted simultaneously, PUCCH format 1 isused, and the ACK/NACK signal is transmitted after being modulated toresources allocated to the SR.

The entire PUCCH formats use cyclic shift (CS) of a sequence for eachOFDM symbol. A base sequence is cyclically shifted by specific CS amountto generate a cyclic shift sequence. The specific CS amount is indicatedby a CS index.

The sequence length is equal to the number of an element included in thesequence. The sequence index for indicating the sequence may bedetermined based on a cell identifier, a slot number within a radioframe, and the like. Assuming that a base sequence is mapped to oneresource block in the frequency domain, one resource block includes 12subcarriers, and thus the length of the base sequence N is 12. Thecyclic shift sequence may be generated by cyclically shifting the basesequence.

The available cyclic shift index to the base sequence may be inducedfrom the base sequence based on a CS interval. For example, when thebase sequence length is 12 and the CS interval is 2, total number of theavailable cyclic shift indices to the base sequence is 6. Hereinafter,HARQ ACK/NACK signal transmission in PUCCH format 1b will be described.

FIG. 5 is a schematic view illustrating a multiple carrier in an LTE-Asystem.

A 3GPP LTE system supports a case where a DL bandwidth and a ULbandwidth are configured differently, in which one component carrier(CC) is required for each of a DL and a UL. The 3GPP LTE system supportsup to 20 MHz, in which the DL bandwidth and the UL bandwidth may bedifferent but one CC is supported for each of the UL and DL.

However, an LTE-A system may support a plurality of CCs through spectrumaggregation (also referred to as bandwidth aggregation or carrieraggregation). For example, when five CCs are allocated as a granularityof a carrier unit with a bandwidth of 20 MHz, the LTE-A system maysupport a bandwidth of up to 100 MHz.

One DL CC or a pair of a UL CC and a DL CC may correspond to one cell.Thus, it is understood that a UE communicating with a base stationthrough a plurality of DL CCs is provided with services from a pluralityof serving cells.

FIG. 5 illustrates three DL CCs and two UL CCs which are subjected tocarrier aggregation. The number of DL CCs and UL CCs subjected tocarrier aggregation is not limited. A PDCCH and PDSCH are independentlytransmitted in each DL CC, and a PDCCH and PDSCH are independentlytransmitted in each UL CC. Two DL CC-UL CC pairs and one DL carrier aredefined, which means that a UE is provided with services from threeserving cells.

The UE may monitor a PDCCH in a plurality of DL CCs and simultaneouslyreceive DL transport blocks through the plurality of DL CCs. Further,the UE may simultaneously transmit a plurality of UL transport blocksthrough a plurality of UL CCs.

A pair of a first DL CC (DL CC #1) and a first UL CC (UL CC #1) may be afirst serving cell, a pair of a second DL CC (DL CC #2) and a second ULCC (UL CC #2) may be a second serving cell, and a third DL CC (DL CC #3)may be a third serving cell. Each serving cell may be identified by acell index (CI). A CI may be unique for a cell or have a UE-specificvalue. Here, for example, the first to third serving cells are allocatedCIs of 0, 1, 2, respectively.

Serving cells may be classified into a primary cell or P-cell and asecondary cell or S-cell. A P-cell may also be referred to as a primarycomponent carrier (PCC), and an S-cell may also be referred to as asecond component carrier (SCC). A P-cell may be designated in an initialconnection establishment procedure, connection reestablishment procedureand handover procedure of a UE. A P-cell may alternatively be referredto as a reference cell. An S-cell may be configured after a radioresource control (RRC) connection is established and be used forproviding additional radio resources. At least one P-cell is alwaysconfigured and an S-cell may be added/revised/cancelled by higher-levelsignaling (for example, an RRC message).

A P-cell may have a fixed CI. For example, a lowest CI may be designatedas a CI of the P-cell. Specifically, the CI of the P-cell may beallocated 0, and CIs of S-cells may be allocated sequential values from1.

The UE may monitor a PDCCH through a plurality of serving cells.However, even in the presence of N serving cells, a base station mayconfigure the UE to monitor PDCCHs of M (M≤N) serving cells. Further,the base station may configure the UE to preferentially monitor PDCCHsof L (L≤M≤N) serving cells.

In performing carrier aggregation in LTE-A, non-cross carrier schedulingand cross-carrier scheduling may be used. In non-cross carrierscheduling, when DL transmission is performed through a particular DLCC, UL transmission may be performed only through a UL CC correspondingto the particular DL CC.

In detail, a DL allocation and a UL grant, transmitted through a PDCCHof a DL CC of a particular cell, may be used for scheduling aPDSCH/PUSCH of the cell to which the DL CC belongs (the cell isconstituted by the DL CC or a UL CC corresponding to the DL CC). Arelationship between the DL CC and the UL CC may be configured throughsystem information block (SIB)-2. That is, a search space that is aregion for detecting the DL allocation and UL grant may be included inthe PDCCH of the cell in which the PDSCH/PUSCH to be scheduled islocated.

In cross-carrier scheduling, a monitored cell may be configured. A DLallocation and a UL grant, transmitted in a PDCCH region of themonitored cell, may be a DL allocation and a UL grant for a cellconfigured to be scheduled in the monitored cell. That is, incross-carrier scheduling, the PDCCH of the monitored cell may transmitresource scheduling information on a plurality of CCs.

In existing 3GPP LTE, although a UE supports a plurality of servingcells, a single timing advance (TA) value is commonly applied to theserving cells. However, when the serving cells are substantially farfrom each other in a frequency domain, propagation properties may changeby serving cells. Also, a remote radio header (RRH) and devices may bepresent in a base station area to extend coverage or remove coverageholes. In this case, since a distance between the base station and theUE and a distance between the RRH and the UE are different from eachother, propagation properties may change.

Hereinafter, uplink time alignment in 3GPP LET is described.

To reduce interference due to uplink transmissions from a plurality ofUEs, it is important for a base station to maintain uplink timealignments of the UEs. A UE may be located at a random place within acell and an arrival time an uplink signal transmitted from a UE takes toreach the base station may vary depending on the location of each UE. AUE located at a cell edge has a longer arrival time than a UE located ina cell center. On the contrary, the UE located in the cell center has ashorter arrival time than the UE located at the cell edge.

To reduce interference by uplink transmissions from a plurality of UEs,it is necessary that a base station schedules uplink signals transmittedby a plurality of UE in a cell to be received within each time boundary.The base station properly adjusts transmission timings of the respectiveUEs to reduce interference in uplink transmission from the plurality ofUEs. Adjusting a transmission timing of a UE by the base station may bereferred to as uplink time alignment.

As one method of uplink time alignment, a UE may perform random access.The UE transmits a random access preamble to the base station. The basestation determines a time alignment value for advancing or delaying atransmission timing of the UE based on the received random accesspreamble. The base station transmits a random access response includingthe determined time alignment value to the UE. The UE may update theuplink transmission timing based on the time alignment value included inthe random access response.

According to another method, the base station may periodically orrandomly receive a sounding reference signal from the UE, determine atime alignment value of the UE through the sounding reference signal,and notify the UE of the determined time alignment value through amedium access control (MAC) control element (CE).

A time alignment value may be information transmitted from the basestation to maintain uplink time alignment of a UE and a timing advancecommand (TAC) transmitted from the base station may include a timealignment value.

A UE generally has mobility, and thus a transmission timing of the UEmay change depending on speed and location of the travelling UE. Thus, atime alignment value received by the UE may be a value valid for aspecific period. A period for which a time alignment value is valid maybe determined based on a time alignment timer.

The UE receives the time alignment value from the base station, updatestime alignment, and then starts or restarts a time alignment timer. TheUE is allowed to perform uplink transmission only when the timealignment timer is in operation. A value of the time alignment timer maybe transmitted by the base station to the UE through system informationor an RRC message, such as a radio bearer reconfiguration message.

When the time alignment timer expires or is not operating, the UEassumes that the UE does not match the base station in time alignmentand does not transmit any uplink single except for a random accesspreamble.

FIG. 6 is a flowchart illustrating a random access process in 3GPP LTE.

As described above, the random access process may be used for a UE toacquire uplink synchronization with a base station or to be allocated anuplink radio resource from the base station.

The UE receives a root index and a physical random access channel(PRACH) configuration index from the base station. There are 64 randomaccess preamble candidates defined by a Zadoff-Chu (ZC) sequence in eachcell and the root index is a logical index for the UE to generate the 64random access preamble candidates.

Transmission of a random access preamble is limited to a particular timeresource and frequency resource in each cell. The PRACH configurationindex indicates a particular subframe and preamble format fortransmitting a random access preamble.

The UE transmits a random access preamble randomly selected (step S610).

The UE selects one of the 64 random access preamble candidates. The UEselects a subframe indicated by the PRACH configuration index. The UEtransmits the selected random access preamble in the selected subframe.

The base station, which receives the random access preamble, transmits arandom access response (RAR) to the UE (step S620).

The RAR is detected in two steps. First, the UE detects a PDCCH maskedwith a random access-RNTI (RA-RNTI). The UE receives the RAR in an MACprotocol data unit (PDU) on a PDSCH indicated by the detected PDCCH.

FIG. 7 illustrates an example of the RAR.

The RAR may include a TAC, a UL grant and a temporary cell-radio networktemporary identifier (C-RNTI).

The TAC may include a time alignment value transmitted by the basestation for UL time alignment of the UE. The UE updates a ULtransmission timing using the time alignment value. When the UE performtime alignment based on the received TAC, a time alignment timer isstarted or restarted. That is, the TAC may include information fortiming adjustment of the UE.

A UL grant transmit may include UL resource allocation information and atransmit power command (TPC). The TPC is used to determine transmitpower for a scheduled PUSCH.

Referring back to FIG. 6, the UE transmits a scheduled message to thebase station according to the UL grant in the RAR (S630).

FIG. 8 is a schematic view illustrating differences in propagationproperties between a plurality of cells.

In existing LTE Release 8/9/10 systems, when a plurality of servingcells is aggregated, a UE performs UL transmission by applying a TAvalue applicable to one cell (for example, P-cell or PCC) commonly tothe plurality of serving cells.

When data transmission and reception between a UE and a base station isperformed based on carrier aggregation, a plurality of serving cellswhich is substantially far from each other in a frequency domain and hasdifferent propagation properties may be aggregated. Further, since aparticular cell among the plurality of serving cells may be used in aremote radio header (RRH), such as a repeater, in order to extendcoverage or remove coverage holes, propagation properties may bedifferent between the serving cells.

When the serving cells have different propagation properties, applying asingle TA value commonly to the plurality of serving cells in ULtransmission by a UE as used in a conventional method may cause adesynchronization of a UL transmission timing for a particular servingcell, thereby leading to time misalignment between the UE and the basestation.

For example, FIG. 8 supposes that a macro base station 800 performs DLtransmission to a UE through a first serving cell and an RRH 820performs DL transmission to the UE through a second serving cell. Indetail, the macro base station 800 may transmit DL data to the UEthrough the first serving cell and the RRH 820 installed due to limitedcoverage may transmit DL data to the UE through the second serving cell.

A propagation delay of the DL data transmitted through the first servingcell may have a different value from a propagation delay of the DL datatransmitted through the second serving cell due to various reasons (forexample, a difference in processing time between the RRH 820 and themacro base station 800 and a difference between a distance from the RRH820 to the UE and a distance from the macro base station 800 to the UE).

When a plurality of serving cells which is carrier-aggregated havedifferent propagation delays, a UE may perform UL transmission based ondifferent TA values for the respective serving cells in performing ULtransmissions through the serving cells with the different propagationdelays. That is, when pieces of DL data transmitted through theplurality of serving cells have different propagation delay properties,the UE may perform UL transmissions based on a plurality of TA values.

FIG. 9 is a schematic view illustrating UL transmission of a UE based ona plurality of TA values.

FIG. 9 illustrates UL transmissions through two serving cells. Apropagation delay for a second serving cell (for example, S-cell) 920may be greater than a propagation delay for a first serving cell (forexample, P-cell) 930.

In this case, a second TA value applied to transmission of second ULdata (for example, second PUSCH data) by a UE through the second servingcell 920 may be greater than a first TA value applied to transmission offirst UL data (for example, first PUSCH data) through the first servingcell 910. When data transmission and reception is performed based oncarrier aggregation, a TA value for each carrier may be applied. TAvalues for a plurality of serving cells may have different from eachother. TA information on each of the serving cells may be transmitted toa UE from a base station corresponding to each serving cell.

When a difference between a first transmission time of first UL datatransmitted by the UE through the first serving cell 910 and a secondtransmission time of second UL data transmitted by the UE through thesecond serving cell is a certain value or greater due to a differencebetween the first TA value and the second TA value, various problems mayoccur. When the difference between the first TA value and the second TAvalue is a certain value or greater, for example, transmission timingrelationship between the base station and the UE is not regular to causemalfunctions of the base station and the UE. Further, when the UEprocesses received DL data and transmits UL data to the base station inresponse to the DL data, complexity increases and a processing time forUL transmission by the UE may be insufficient.

Hereinafter, one embodiment of the present invention discloses an ULtransmission method of a UE when the UE receives TA values correspondingto the respective serving cells (for example, the first serving cell 910and the second serving cell 920) and a difference between the first TAvalue for the first serving cell 910 and the second TA value for thesecond serving cell 920 is a threshold or greater. The threshold may beset in the UE through a higher signal or be recognized in advance by theUE. In the embodiment of the present invention, when the differencebetween the TA values of the respective serving cells is the thresholdor greater when the UE performs UL transmission, the UE may drop UL datatransmission of the UE or limit a UL transmission timing of the UE. Thefollowing embodiment of the present invention discloses a detailedoperation of the UE when the difference between the TA values is thethreshold or greater.

In the following embodiment, the difference between the TA values forthe plurality of serving cells may be interpreted variously. A TA valuemay be a value representing how much UL transmission precedes a DLreception time of the UE in a time domain. Reception times at which theUE receives DL data from the respective serving cells or boundaries ofDL subframes may not be the same. Thus, a reference time for calculatinga TA value for each serving cell may vary depending on serving cells.When the difference between the TA values for the respective servingcells is simply calculated without considering a DL data reception time,the difference between the TA values may be a value reflecting even adifference between the reception times of DL data that the UE receivesthrough respective carrier components.

The difference between the TA values for the plurality of serving cellsillustrated in the embodiment of the present invention may be a TA valuedifference which reflects even the difference between the receptiontimes of DL data or a value obtained in consideration of only adifference between transmission timings of UL data transmitted throughthe respective serving cells when the UE transmits a UL subframe throughthe respective serving cells. Here, a TA value for a particular cell maymean merely a transmission timing of the UE in the cell. A TA differencein the embodiment of the present invention may also be interpreted asthe difference between the TA values for the respective serving cellsreceived by the UE from the base station, a difference in transmissiontimings to be applied by the UE in transmission, or a difference betweenTA values derived based on a TAC received by the UE. Signal transmissionin which TA application managed through a TAC value is excluded, such asa PRACH, may not be subjected to a TA difference limitation method to bementioned. In the following embodiments of the present invention, forconvenience of description, a TA value difference will be described onthe assumption that DL data reception times as a reference for derivingthe TA value difference are the same in a plurality of serving cells.

FIG. 10 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 10 illustrates a method of dropping a UL signal when a differencebetween TA values for a plurality of serving cells is a threshold orgreater.

Whether the difference between the TA values for the plurality ofserving cells is the threshold or greater may be determined based on aTA value for a particular serving cell (reference TA value). Accordingto the embodiment of the present invention, it may be determined whethera difference between the reference TA value and a TA value for anotherserving cell carrier-aggregated with the particular serving cell is thethreshold or greater. When the difference between the reference TA valueand the TA value for the other serving cell is the threshold or greater,UL transmission performed through the other serving cell may be dropped.The serving cell for determining the reference TA value may be apredetermined serving cell (for example, a PCC). Alternatively, theserving cell for determining the reference TA value may be set through ahigh-layer signal, such as RRC signaling.

Referring to FIG. 10, a first TA value for a first serving cell 1010 maybe set as the reference TA value and a difference between the referenceTA value and a second TA value for a second serving cell 1020 may be thethreshold or greater. In this case, UL data (for example, second PUSCHdata) transmitted through the second serving cell may be dropped.

Dropping UL transmission in a serving cell may mean an operation of theUE not transmitting UL data (for example, a periodic channel qualityindicator (CQI)) configured to be transmitted in advance in a servingcell or an operation of the UE not expecting or neglecting a ULscheduling command for a serving cell.

FIG. 11 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 11 illustrates a method of dropping a UL signal of a particular TAgroup when a plurality of serving cells is classified into different TAgroups and a difference between TA values for the TA groups is athreshold or greater.

Referring to FIG. 11, a first serving cell 1110 and a second servingcell 1120 may be classified into a first TA group 1100, and a thirdserving cell 1130 and a fourth serving cell 1140 may be classified intoa second TA group 1150. The same TA group may be a group with a TAdetermined based on the same TAC. A TA for the first TA group 1100 maybe set as a first TA value and a TA for the second TA group 1150 may beset as a second TA value.

Whether the difference between TA values for the plurality of TA groupsis the threshold or greater may be determined based on a TA value for aparticular TA group (reference TA value). According to the embodiment ofthe present invention, it may be determined whether a difference betweenthe reference TA value and a TA value for another TA groupcarrier-aggregated with the particular TA group is the threshold orgreater. When the difference between the reference TA value and the TAvalue for the other TA group is the threshold or greater, ULtransmission performed through the other TA group may be dropped. The TAgroup for determining the reference TA value may be a TA group includinga predetermined serving cell (for example, a PCC). Alternatively, the TAgroup for determining the reference TA value may be set through ahigh-layer signal, such as RRC signaling.

Referring to FIG. 11, the first TA value for the first TA group 1100 maybe set as the reference TA value and it may be determined whether adifference between the reference TA value and the second TA value forthe second TA group 1150 is a threshold or greater. When the differencebetween the first TA value as the reference TA value and the second TAvalue is the threshold or greater, UL data (for example, third PUSCHdata and fourth PUSCH data) transmitted through the serving cellsincluded in the second TA group 1150, the third serving cell 1160 andthe fourth serving cell 1170, may be dropped.

FIG. 12 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

In FIG. 12, when a difference between TA values for a plurality ofcarrier components is a threshold or greater, UL transmission may beperformed by adjusting the difference between the TA values to be thethreshold or less.

Referring to FIG. 12, a difference between a first TA value for a firstserving cell 1210 and a second TA value for a second serving cell 1220may be the threshold or greater.

Likewise, whether the difference between the TA values for the pluralityof serving cells is the threshold or greater may be determined based ona TA value for a particular serving cell (reference TA value). Accordingto the embodiment of the present invention, it may be determined whethera difference between the reference TA value and a TA value for anotherserving cell carrier-aggregated with the particular serving cell is thethreshold or greater. When the difference between the reference TA valueand the TA value for the other serving cell is the threshold or greater,the UE may perform UL transmission through the other serving cell basedon an adjusted TA value by adjusting the TA value for the other servingcell. The adjusted TA value may be determined such that a differencebetween the reference TA value and the adjusted second TA value is thethreshold or less.

The serving cell for determining the reference TA value may be apredetermined serving cell (for example, a PCC). Alternatively, theserving cell for determining the reference TA value may be set through ahigh-layer signal, such as RRC signaling.

Referring to FIG. 12, a first TA value for the first serving cell 1210may be set as the reference TA value and a difference between thereference TA value and a second TA value for the second serving cell1220 may be the threshold or greater. In this case, the second TA valuefor the second serving cell 1220 may be adjusted to an adjusted secondTA value. The adjusted second TA value may be determined such that adifference between the reference TA value and the changed second TAvalue is the threshold or less.

FIG. 13 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 13 illustrates a method of performing UL transmission by adjustinga TA value for a particular TA such that a TA value difference is athreshold or less when a plurality of serving cells is classified intodifferent TA groups and the difference between the TA values for the TAgroups is the threshold or greater.

Referring to FIG. 13, a first serving cell component 1310 and a secondserving cell 1320 may be classified into a first TA group 1300, and athird serving cell 1330 and a fourth serving cell 1340 may be classifiedinto a second TA group 1350. The same TA group may be a group with a TAdetermined based on the same TAC. A TA for the first TA group 1300 maybe set as a first TA value and a TA for the second TA group 1350 may beset as a second TA value.

Whether the difference between the TA values for the plurality of TAgroups is the threshold or greater may be determined based on a TA valuefor a particular TA group (reference TA value). According to theembodiment of the present invention, it may be determined whether adifference between the reference TA value and a TA value for another TAgroup carrier-aggregated with the particular TA group is the thresholdor greater. When the difference between the reference TA value and theTA value for the other TA group is the threshold or greater, ULtransmission may be performed through a serving cell corresponding tothe other TA group based on an adjusted TA value by adjusting the TAvalue for the other TA group. The adjusted TA value may be determinedsuch that a difference between the reference TA value and the adjustedsecond TA value is the threshold or less.

The TA group for determining the reference TA value may be a TA groupincluding a predetermined serving cell (for example, a PCC).Alternatively, the TA group for determining the reference TA value maybe set through a high-layer signal, such as RRC signaling.

Referring to FIG. 13, the first TA value for the first TA group 1300 maybe set as the reference TA value and it may be determined whether adifference between the reference TA value and the second TA value forthe second TA group 1350 is a threshold or greater. When the differencebetween the first TA value as the reference TA value and the second TAvalue is the threshold or greater, the second TA value for the servingcells included in the second TA group 1350, the third serving cell 1360and the fourth serving cell 1370, may be adjusted to an adjusted secondTA value. The UE may transmit UL data (for example, third PUSCH data andfourth PUSCH data) through the third serving cell 1160 and the fourthserving cell 1370 based on the adjusted second TA value.

FIG. 14 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

In FIG. 14, when a difference between TA values for a plurality ofcarrier components is a threshold or greater, a TAC transmitted from thebase station may be neglected.

Likewise, whether a difference between TA values for a plurality ofserving cells is the threshold or greater may be determined based on aTA value for a particular serving cell (reference TA value). Accordingto the embodiment of the present invention, it may be determined whethera difference between the reference TA value and a TA value for anotherserving cell carrier-aggregated with the particular serving cell is thethreshold or greater. When the difference between the reference TA valueand the TA value for the other serving cell is the threshold or greater,the UE may neglect the TAC received from the base station. The UE mayperform UL transmission based on a random TA value without determiningthe TA value for the other serving cell based on the received TAC. Therandom TA value may be expressed as a term “TA value determined by theUE.”

The serving cell for determining the reference TA value may be apredetermined serving cell (for example, a PCC). Alternatively, theserving cell for determining the reference TA value may be set through ahigh-layer signal, such as RRC signaling.

Referring to FIG. 14, a first TA value for a first serving cell 1410 maybe set as the reference TA value and a difference between the first TAvalue for the first serving cell 1410 and a second TA value for a secondserving cell 1420 which are received from the base station may be thethreshold or greater. In this case, the UE may neglect the TAC includinginformation on the second TA value. The UE may perform UL transmissionthrough the second serving cell 1420 using a TA value determined by theUE without considering the received second TA value. The TA valuedetermined by the UE may be a TA value used by the UE in previous ULtransmission or be a value adjusted to have a difference from the firstTA value within the range of the threshold as in FIGS. 12 and 13.

FIG. 15 is a schematic view illustrating an uplink transmission methodaccording to an embodiment of the present invention.

FIG. 15 illustrates a method of performing UL transmission by adjustinga TA value for a particular TA such that a TA value difference is athreshold or less when a plurality of serving cells is classified intodifferent TA groups and the difference between the TA values for the TAgroups is the threshold or greater.

Referring to FIG. 15, a first serving cell component 1510 and a secondserving cell 1520 may be classified into a first TA group 1500, and athird serving cell 1530 and a fourth serving cell 1540 may be classifiedinto a second TA group 1550. The same TA group may be a group with a TAdetermined based on the same TAC. A TA for the first TA group 1500 maybe a first TA value and a TA for the second TA group 1550 may be asecond TA value.

Whether the difference between the TA values for the plurality of TAgroups is the threshold or greater may be determined based on a TA valuefor a particular TA group (reference TA value). According to theembodiment of the present invention, it may be determined whether adifference between the reference TA value and a TA value for another TAgroup carrier-aggregated with the particular TA group is the thresholdor greater. When the difference between the reference TA value and theTA value for the other TA group is the threshold or greater, a TACincluding information on the TA value for the other TA group may beneglected. The UE may perform UL transmission based on a TA valuedetermined by the UE.

The TA group for determining the reference TA value may be a TA groupincluding a predetermined serving cell (for example, a PCC).Alternatively, the TA group for determining the reference TA value maybe set through a high-layer signal, such as RRC signaling.

Referring to FIG. 15, the first TA value for the first TA group 1500 maybe set as the reference TA value and it may be determined whether adifference between the reference TA value and the second TA value forthe second TA group 1550 is a threshold or greater. When the differencebetween the first TA value as the reference TA value and the second TAvalue is the threshold or greater, the UE may neglect a TAC includinginformation on the second TA value. The UE may perform UL transmissionthrough the serving cells in the second TA group 1550 (the third servingcell 1560 and the fourth serving cell 1570) using the TA valuedetermined by the UE without considering the received second TA value.The TA value determined by the UE may be a TA value used by the UE inprevious UL transmission or be a value adjusted to have a differencefrom the first TA value within the range of the threshold as in FIGS. 12and 13.

FIG. 16 is a block diagram showing a wireless communication systemaccording to an embodiment of the present invention.

Referring to FIG. 16, a BS 1600 includes a processor 1610, a memory 1620and a RF unit 1630. The memory 1620 is connected to the processor 1610and configured to store various information used for the operations forthe processor 2010. The RF unit 2030 is connected to the processor 1610and configured to transmit and/or receive a radio signal. The processor1610 implements the proposed functions, processed, and/or methods. Inthe described embodiments, the operation of BS may be implemented by theprocessor 1610.

For example, the processor 1610 may be configured to transmit a TAC fordetermining a timing of an uplink subframe to the UE.

A wireless apparatus 1650 includes a processor 1660, a memory 1670, anda radio frequency (RF) unit 1680. The memory 1670 is connected to theprocessor 1660 and configured to store various information used foroperating the processor 1660. The RF unit 1680 is connected to theprocessor 1660 and configured to transmit and/or receive a radio signal.The processor 1660 implements the proposed functions, processed, and/ormethods. In the embodiments described above, the operation of thewireless apparatus may be implemented by the processor 1660.

For example, the processor 1660 may be configured to receive a first TACfor a first serving cell and a second TAC for a second serving cell andto determine whether to transmit uplink data through the second servingcell based on whether a timing difference is a threshold or less. Here,the timing difference may be acquired based on the first TAC and thesecond TAC and the first serving cell may be configured to alwaysperform uplink transmission regardless of the timing difference.

The processor may include application-specific integrated circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include read-only memory (ROM), random access memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for performing uplink transmission, themethod performed by a user equipment (UE), which is configured with afirst cell and a second cell, the method comprising: receiving a firsttiming advance command for the first cell and a second timing advancecommand for the second cell, wherein the first timing advance command isinformation for a transmission timing of the first cell, and the secondtiming advance command is information for a transmission timing of thesecond cell; performing the uplink transmission based on the timingdifference between the first cell and the second cell, wherein thetiming difference is determined based on the first timing advancecommand and the second timing advance command, wherein the UE furtherreceives reference cell information related to whether a reference cell,at which a transmission timing is fixed, is the first cell or the secondcell in case that the timing difference exceeds a threshold value,wherein the reference cell information is transmitted through a radioresource control (RRC) layer, wherein, if the reference cell is thefirst cell and the timing difference between the first cell and thesecond cell exceeds the threshold value, the UE adjusts the timingdifference to be within the threshold value by adjusting thetransmission timing of the second cell, and wherein, if the referencecell is the second cell and the timing difference between the first celland the second cell exceeds the threshold value, the UE adjusts thetiming difference to be within the threshold value by adjusting thetransmission timing of the first cell.
 2. The method of claim 1,wherein, if the timing difference between the first cell and the secondcell does not exceed the threshold value, the UE performs the uplinktransmission on the first cell based on the first timing advance commandand performs the uplink transmission of the second cell based on thesecond timing advance command.
 3. The method of claim 1, wherein thefirst timing advance command is information for a first timing advancegroup including the first cell, and the second timing advance command isinformation for a second timing advance group including the second cell.4. A user equipment (UE), which is configured with a first cell and asecond cell, comprising: a radio frequency (RF) unit configured totransmit and receive a radio signal; and a processor operativelyconnected with the RF unit and configured to: receive a first timingadvance command for the first cell and a second timing advance commandfor the second cell, wherein the first timing advance command isinformation for a transmission timing of the first cell, and the secondtiming advance command is information for a transmission timing of thesecond cell; and perform uplink transmission based on a timingdifference between the first cell and the second cell, wherein thetiming difference is determined based on the first timing advancecommand and the second timing advance command, wherein the UE furtherreceives reference cell information related to whether a reference cell,at which a transmission timing is fixed, is the first cell or the secondcell in case that the timing difference exceeds a threshold value,wherein the reference cell information is transmitted through a radioresource control (RRC) layer, wherein, if the reference cell is thefirst cell and the timing difference between the first cell and thesecond cell exceeds the threshold value, the UE adjusts the timingdifference to be within the threshold value by adjusting thetransmission timing of the second cell, and wherein, if the referencecell is the second cell and the timing difference between the first celland the second cell exceeds the threshold value, the UE adjusts thetiming difference to be within the threshold value by adjusting thetransmission timing of the first cell.
 5. The UE of claim 4, wherein, ifthe timing difference between the first cell and the second cell doesnot exceed the threshold value, the UE performs the uplink transmissionon the first cell based on the first timing advance command and performsthe uplink transmission of the second cell based on the second timingadvance command.
 6. The UE of claim 4, wherein the first timing advancecommand is information for a first timing advance group including thefirst cell, and the second timing advance command is information for asecond timing advance group including the second cell.